Narrow-linewidth semiconductor lasers have been proposed for use as optical sources in FM laser ranging systems. In such a system, an optical beam is transmitted towards a target, and the reflection from the target is received and mixed with a sample of the transmitted beam to produce a detection signal. The frequency of the transmitted beam is swept in a linear manner, such that the frequency of the detecting signal is measure of target range. For intermediate range (10-1,000 meters) and high precision (10-100 microns) systems, a laser source is required having a narrow linewidth, and having the capability of being rapidly tuned over a wide frequency range. To date, there have been no suitable techniques available for adapting semiconductor lasers for use in such systems.